Impact of Temperature on Cherenkov Photon Generation from Radiotherapy Beams
Abstract
Purpose
Cherenkov radiation imaging has become an increasingly more common imaging modality as a real-time beam delivery visualization aid with potential for dosimetry. Dosimetric studies require a comprehensive analysis of factors which affect the optical photon generation and propagation, both of which are dependent on the medium’s index of refraction. To our knowledge, this is the first study to address the impact of temperature and refractive index on the Cherenkov signal generated in a theoretical water tank model, supported by experimental results.
Methods
The impact of temperature on Cherenkov photon generation in water was estimated by first using the Lorentz-Lorenz relation to calculate changes in index of refraction with temperature. When combined with the energy threshold for Cherenkov photon generation, a theoretical framework providing a baseline estimate established. Experimentally, a water tank was heated to temperatures ranging from 70°F to 130°F, representing a relevant clinical/ ambient temperature range. The tank was irradiated with 6MV and 15MV photons at each temperature step, The average optical signal recorded in the exit beam geometry across all 30 measurements was compared over a common bounding region.
Results
The water tank model results showed that the optical signal measured by the detector decreased as the temperature increased. From 70°F to 130°F, the theoretical model predicted an approximately 2.05% decrease in Cherenkov photon generation. The experimental results from 6MV and 15MV cases found a 3.13% and 2.09% decrease over the same range, respectively.
Conclusion
There exists a strong agreement between the theoretical and experimental results in a water tank model which highlights a decrease in Cherenkov photon generation over increasing temperature. This effect, while possibly necessary to account for in applications which may require high accuracy photon counting, is unlikely to cause any discernable impact on interpretability over clinically relevant temperatures.